[simonask]

It is essential and nontrivial, but it's also the extremely bare minimum.

You cannot write portable code without platform-specific and even environment-specific adaptations, like handling the presence of certain headers (looking at you, stdint.h and stddef.h), and let's not even start about interacting with the OS in any way.

[uecker]

There may be platforms that are not conforming to the C standard. But I doubt those then have comprehensive implementations of other languages either.